A STUDY OF IRON IN FIRED CLAY : MÖSSBAUER EFFECT AND MAGNETIC MEASUREMENTS

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A STUDY OF IRON IN FIRED CLAY : MÖSSBAUER

EFFECT AND MAGNETIC MEASUREMENTS

R. Chevalier, J. Coey, R. Bouchez

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplhnent au no 12, Tome 37, Dkcembre 1976, page C6-861

A STUDY OF IRON IN FIRED CLAY

:

M~SSBAUER EFFECT

AND MAGNETIC MEASUREMENTS

R. CHEVALIER

DRF/Groupe d'Hyperfines, Centre d'Etudes NuclCaires de Grenoble 85 X, 38041 Grenoble Cedex, France J. M. D. COEY

Groupe des Transitions de Phases, Centre National de la Recherche Scientifique R. BOUCHEZ

Laboratoire de Recherche ArchCologique, Institut des Sciences Nucleaires 257 X, 38044 Grenoble Cedex, France

R6sum6.

-

Une terre de potier, provenant de la plaine de Gorgan au N-E de I'Iran, a et6 cuite suc~essivement de 200 a la fusion 1 200 "C, soit dans I'air, soit dans un melange reducteur (5 % CO dans CO2 humide). Les spectres Mossbauer mesures B 296 K et 4,2 K permettent de determiner la proportion relative du fer soit dans I'hematite ou la magnetite bien cristallises, soit dans les oxydes ou hydroxydes ferriques a grains fins ou peu cristallis6s, soit du fer 2+ ou 3 + disperse dans les silicates. Dans une cuisson oxydante il n'y plus d'ions Fez+ au-dessus de 500 "C ; et B

1 200 "C tous les ions Fe3f sont disperses dans les silicates. Un pic d'aimantation ( - 5 uem/g. Fe) apparait ii 1 100 "C associk avec la diminution de la taille des grains fins d'oxyde ferrique avant leur dispersion B 1 200 "C dans les silicates. Dans une cuisson reductrice, des ions Fe3+ semblent sub- sister jusqu'k

-

1 200 "C,

-

12 % de fer existe dans la phase magnetite entre 600 et 800 "C

puis disparait rapidement dbs 850 "C, ce qui rend compte de la chute rapide de I'aimantation depuis

-

16 uem/g. Fe B 800 "C B 0 B 1 100 "C.

Abstract. - A potters clay from the Gorgan plain, in N. E. Iran, has been fired successively at temperatures up to the melting point 1 200 "C in an atmosphere of air or 5 % CO in moist COz.

Mossbauer spectra at 296 K and 4.2 K allow the determination of the relative amounts of iron in well-crystallised haematite or magnetite, of poorly-crystallised and fine-particle ferric oxide or hydroxide, and of iron diluted in the ferrous or ferric form in silicate minerals. In an oxidizing atmosphere there is no ferrous iron in firings above 500 "C and at 1 200 "C, all the ferric iron is diluted in the silicate matrix. A peak in the magnetization which reaches 5 emufg. Fe near 1 100 "C

is associated with a decrease in the size of ferric oxide fine particles before their disappearance at 1 200 "C. In a reducing atmosphere, some ferric iron persists up to about 1 200 OC, 12 % of the iron is in the form of magnetite between 600 and 800 "C, and its progressive disappearance from 850 "C accounts for the drop in magnetization from 16 emulg. Fe at 800 OC to zero at 1 100 "C.

1. Introduction.

-

Fired clay is among mankind's most ancient and most useful manufactures. Al- though an excellent empirical understanding of the firing process has been developed over the ages, a complete fundamental understanding of the complex chemical and mineralogical transformations induced by firing clay has yet to be achieved.

In this paper, we fix our attention on the behaviour of just one element during the firing process - the iron. It is the fourth most abundant element in the Earth's crust, and a major constituent of most clay- grade minerals. The two main measurements reported here, 57Fe Mossbauer spectra and bulk magnetic properties, detect only the iron, the latter because iron is by far the most abundant magnetic ion in clay. Applications of the Mossbauer effect in the study

of the clay minerals have recently been reviewed by on of the authors [l]. The work on heat treatments has almost all involved pure minerals, treated in air. However Simopoulos et al. have performed a Moss-

bauer study on an air-fired Attic clay containing a mixture of minerals whose scope resembles that of the present work 121. Our clay is perhaps more repre-

sentative in that most of iron present is diluted in the clay minerals, rather than in the form of iron oxides and hydroxides. We have studied the transformations induced by firing both in oxidizing and reducing atmo- spheres, and relate the Mossbauer results to the magnetization data which was reported in some detail in reference [3]. Use of the two techniques should allow a complete account of the transformations involving the iron to be developed.

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C6-862 R. CHEVALIER, J. M. D. COEY AND R. BOUCHEZ

The present work, like that of reference [2], was partly inspired by application of the Mossbauer effect to archaeological problems, and some of the impli- cations in this domain will be discussed.

2. Results.

-

The clay examined was a reddish-pink used currently for pottery manufacture by the potter of Gorgan a town in north-east Iran. Its elemental composition was as follows : SiO, 46.7

%,

CaO 13.4

%,

A1203 12.1

%,

F e 0

+

Fe,03 4.5

%,

MgO 3.2%, K,O 2.4%, N a 2 0 1.4%, TiO, 0.6%. The weight lost on ignition was 16.6

%.

The mineral composition determined by X-ray diffraction, was Quartz 30

%,

Calcite 20

%,

Feldspar 10

%,

Illite 20

%,

Chlorite 10

%,

Smectite and Kaolinite 10

%.

Its Mossbauer parameters at room temperature was previously reported in [4] (C2). Ferrous iron accounts for 29

%

of the total absorption and from the isomer shift and quadrupole splitting, it was concluded that this ferrous iron was mainly present in the Chlorite. Out of the remaining 71

%

of ferric absorbtion 12% was due to well-crystallised Haematite, another 21

%

to other concentrated iron oxide or hydroxide phases, possibly coatings on the clay mineral particles, which order or block magnetically in the range 4-300 K, and the remaining 38

%

to iron in the clay minerals themselves, mainly the Illite and Smectite Error are

f 3

%.

If equal recoilless fractions can be assumed for all the iron, these figures represent the relative amounts of iron in each phase.

Two sets of firings were studied in detail. Separate samples were fired at each temperature, in one case in air, in the other in an atmosphere of 5

%

CO in moist CO,. 1 g samples of the dry clay were heated up over five hours, kept at constant temperature T, for ten hours and then cooIed in the furnace over ten hours.

Mossbauer spectra were taken at room temperature and at liquid helium temperature on all the samples, and magnetic hysteresis loops were measured at room temperature. Some typical data are shown in figure 1. The spectra were least-squares fitted with up to two magnetic hyperfine spectra, and two ferrous and one ferric quadrupole doublets. The aim was not so much to fit a subspectrum to every pos- sible iron site, as to reproduce accurately the shape of the spectrum and thus determine the relative absorption of ferrous and ferric iron in both the paramagnetic and magnetically ordered states.

2.1 OXIDIZING FIRINGS. - All trace of ferrous iron had disappeared from the Mossbauer spectra of samples fired in air above 400 OC. The relative intensity of the ferrous quadrupole doublet at 4.2 K was only half of what it was at 296 K. This cannot plausibly be described as a recoilless fraction effect, but is probably due to magnetic ordering of Chlorite at very low temperatures. The Chlorite contains about 10 wt

%

of iron, and magnetic order at 4.2 K is

FIG. 1. - Mijssbauer spectra of clay fired in an oxidizing atmosphere (air) at 925 'C (a and b) or in a reducing atmosphere (a mixture of 5 % CO in moist COz) at 750 "C (c and d). a and c are taken at room temperature whereas b and d are taken

at 4.2 K.

known in other silicates with similar amounts of iron 15, 61.

The well-crystallised ferric oxide which gives a sharp magnetic hyperfine spectrum at 296 K is essentially Haematite. It reaches maximum of 26

%

of the total absorption in the 800OC firing, but disappears completely at 1200 OC. At 4.2 K its hyperfine field H,, is 530 kOe. It is quite distinct from the

second six-line spectrum with broader lines and

H,, 465 kOe which appears in the spectra at 4.2 K but not at 296 K (Fig. la, b). This spectrum is attributed to ferric hydroxides or ferric oxide fine particles which block between 4.2 and 300 K. It also disappears in the clay fired at 1 200 OC.

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FIG. 2. - Relative Mossbauer absorbtion of iron in different phases as a function of firing temperature in an oxidizing atmosphere. o Magnetically-ordered ferric iron a t 296K, Magneti- cally ordered ferric iron at 4.2 K, X total ferric ion at 296 K.

The numbered regions refer to i) well-crystallised haematite, ii) poorly crystallised or fine-particle ferric oxide and hydroxide, iii) ferric iron dilute in silicate phases, iv) ferrous iron dilute

in silicate phrases.

2.2 REDUCING FIRINGS.

-

The behaviour of the iron is completely different when a reducing atmosphere is used\,for the firings. The relative absorption of the ferrous quadrupole doublets in the room temperature spectra increases steadily with firing temperature, and extrapolates to 100

%

near 1 200 0C. At 4.2 K, some of this ferrous absorption has passed into a poorly-defined hyperfine spectrum, suggesting magnetic order in the more iron rich parts of the silicate matrix.

The well-crystallised haematite present in the unfir- ed clay appears to have converted to well-crystallised magnetite by 6000C. It continues to account for approximately 10

%

of the total absorption until fired at 800 OC, but by 950 OC it has essentially disappeared. The intensity of the ferric quadrupole doublet in the room-temperature spectra decreases steadily with firing temperature, and extrapolates to zero at 1 200 OC. These results are summarised on figure 3.

FIG. 3. - Relative Mossbauer absorption of iron in different phases as a function of firing temperature in a reducing atmosphere, X ferrous iron paramagnetic a t 296 K,o total iron paramagnetic at 296 K. The lettered regions refer to a) ferrous iron dilute in silicate phases, b) ferric iron dilute in silicate phases,

c) magnetite, d ) haematite.

We conclude this section with a remark about the colour of the fired clay. The starting material was pink when dry. It turns red then buff when fired in air up to 1 050 OC, but at higher temperatures it becomes pale yellow. When fired in the reducing conditions, it turns grey, but the grey becomes progressively paler as the firing temperature is increased beyond 800 OC.

3. Discussion. - In general terms, three major transformations occur successively when clay is heated in air : dehydroxylation, vitrification and recrystallization. The first is signaled in the Mossbauer spectrum by a large increase in the ferric quadrupole splitting 12, 8, 91 which occurs at

--

4000C in our clay. Vitrification is the reaction which is important for pottery production. It usually occurs progressively over an extended temperature range which depends sensitively on the chemical composition especially Ca and Fe2' and is best observed directly in the scann- ing electron microscope 110, 71. The decrease in the ferric quadrupole splitting which occurs above about 850OC in our clay [4] may also be associated with vitrification. Recrystallization of the vitreous silicate matrix normally takes place at a higher temperature provided melting does not occur first.

The results we have obtained for the iron in our clay fired in an oxidizing atmosphere (Fig. 2) indicate that the ferrous iron is all oxidized to the ferric form near the temperature where dehydroxylation occurs. Ferrous iron in pure chlorites is known to be completely oxidised by heating in air at 400-500 OC 1111. Also at about 500 OC, the quantity of well-crystallised ferric oxide begins to increase as the haematite in the original clay is supplemented by haematite pro- duced from the poorly crystallised ferric hydroxide, or else by agglomeration of oxide fine particles. Note that the haematite and magnetite refered to in this paper are not necessarily the pure oxides a-Fe,O, or Fe,O,, but may have considerable amounts of cation substitution, e. g. AI3+, Ti4+.

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C6-864 R. CHEVALIER, J. M. D. COEY AND R. BOUCHEZ

The spontaneous magnetization of the fired clay in oxidizing conditions measured at 296 K is shown in figure 4a. Its value throughout, and particularly at the peak at 1 100 OC is far too great to be explained by the amount of haematite present for which

MS 1:0.5 emu/g of Fe

.

FIG. 4. - Spontaneous magnetization a t 296 K of clay fired in a) an oxidizing atmosphere and b) a reducing atmosphere.

MS is given per gram of iron.

The magnetization is likely to come mainly from to fine particles of ferric oxide for which MS may be typically 1-10 emu/g of Fe, but depends strongly on the particle size, being proportional to N-'", where N is the number of iron ions in the particle [13]. A reduction in the average particle size near the peak at 1 100 OC is consistent with the increase in the relative amount of fine particles above 900 OC shown

I I I I l

0 COO 800 1200

T/C

FIG. 5. - The ratio of fine particle to well-crystallised ferric oxides as a function of firing temperature in an oxidizing atmo-

sphere.

in figure 5. One can imagine that the surface atoms of the fine particles of ferric oxide are being dissolved into the recrystallised silicates in this temperature range, thereby reducing the average particle size. Magnetite formation is unlikely for firings in air at these temperatures [7], and it would not produce a peak in Ms.

In the firings in the reducing atmosphere the ferric iron is less easy to reduce than the ferrous iron was to oxidize in the firings in air. The most interesting feature is perhaps the existence of magnetite a t temperatures up to about 8500C. The Mossbauer data shows that

-

12

%

of the iron is in that form, at least between 600 and 800 OC, and with a moment

M S 1:100 emu/g, the high value of the magnetiza-

tion (Fig. 4b) in this range of temperature is nicely accounted for. The fall of the magnetization near 850 OC corresponds to the disappearance of magnetite from the Mossbauer spectrum. It is presumably dissolved in the vitreous silicate matrix, as Fe2+ is soluble in silicate glasses, and is even a glass former. All the ions are diluted in the silicate phases by about l 000 OC in the reducing atmosphere as compared with 1 200 OC in air.

Two other conclusions may be drawn from this work. One concerns the colour of fired clay [l41 which can be directly associated with the iron oxides. For the firings in air the product is reddish-pink or buff so long as haematite is present,

Tf

5

1 000 in the case of our clay, but at

Tf

N 1 200 OC, where the

ferric oxide is absent, the colour is a pale yellow ;

finally at the melting point E 1 230 OC, the colour

is dark brown. For the firings in the CO-CO,mixture, the product is dark grey so long as magnetite is present,

Tf

< 900 OC, in the case of our clay, but at

Tf

2

950 OC the colour becomes a much paler light grey.

The other concerns the use of the proportion of iron in a sherd present in the form of oxide as an indication of firing conditions or age of an ancient ceramic [15]. Our firings in air have demonstrated that this parameter may be a sensitive function of firing temperature, and comparison with the results of a set of firings under slightly different oxidizing conditions (it was kept at the maximum temperature for one hour instead of ten) [l, 31 shows that it depends also on the firing cycle. It would seem to be rather difficult to unravel these effects from those of time, suggested in references [15], unless the original firings are made in a range of temperature or firing conditions to which the proportion of iron in the oxide phase is rather insensitive. This condition appears to be fulfilled around 800 OC in our clay, but not around

1 000 OC.

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A STUDY OF IRON IN FIRED CLAY : MOSSBAUER EFFECT AND MAGNETIC MEASUREMENTS C6-865

which we have described may be qualitatively typical Acknowledgements.

-

W e are grateful to Nguyen of moderately-weathered clays, the same quantitative van Dang for making the magnetic measurements, behaviour cannot be expected regardless of the mine- to Osamu Ogawa f o r help with the data analysis, ralogical a n d chemical composition of the starting a n d to Mrs. M. B. S6le f o r her kind assistance with the

material. sample preparation.

References

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Mossbauer Spectroscopy, Cracow 1975, Vol. 2, p. 333. I21 SIMOPOULOS, A., KOSTIKAS, A., SIGALAS, I., GANGAS, N. H. and MOUKARIKA, A., Clays and Clay Miner. 23 (1975) 393.

131 COEY, J. M. D., BOUCHEZ, R., DANG, N. V. and DES- HAYES, J. Proceedings of the International Sympo- sium on Archaeometry, Edinburgh 1976 (in press).

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J. Physique Colloq. 35, (1974) C6-541.

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